Higher mass effects in heavylight

[felixhekhorn]

To open another non-trivial point: since the whole point of NumFONLL is to account for simultaneous mass-effects we could/should make also heavylight more complicated and account for "higher"-mass effects: i.e. bottom-mass effects to F2c in FFNS4. This can be done using the icoupl argument of generate_missing (and similar for asy)

Originally posted by @felixhekhorn in #195 (comment)